Anyone have an opinion on whether nets training with a very heavy bat is good or bad for technique? I heard it can ruin your timing of the ball...
Based on my experience, there is no benefit in using a heavy bat in the nets. The purpose of "nets" is to develop muscle for all your batting movements. If you think a heavy bat will somehow make you a strong hitter of a ball, it probably will not work unless you club 1000s of balls pre session. If you want to get stronger as a hitter, hit the gym and lift weights.
For nets, I use a bat that weighs the same as my match bat and has the same shape. Sometimes, I'd even use my match bat if I have been practicing with a different shape bat.
Regarding weight and age (related strength and endurance), try out different bat weights and shapes before settling on your ideal weight and shape. Light bats are useless for me but that's just my preference. Your situation could be different. Also, a beefed up middle of a light weight bat might pick up differently and could change your batting experience as well. I would avoid custom bat(s) unless you know a shape that works like gangbusters for you. If not, experiment with as many off-the-shelf shapes as possible and find one that helps hit a ball cleanly without much effort (magic of timing). Have you tried Newbery Blitz shape? You can generate a lot of bat speed with that shape, it is beautiful!
Good luck.